The present invention relates to the field of two-way wireless communication systems and more specifically to methods and apparatus for communication with mobile telephone users (cellular and personal communication systems), basic exchange telecommunications radio, wireless data communications, two-way paging and other wireless systems.
Conventional Cellular Systems
Present day cellular mobile telephone systems developed due to a large demand for mobile services that could not be satisfied by earlier systems. Cellular systems "reuse" frequency within a group of cells to provide wireless two-way radio frequency (RF) communication to large numbers of users. Each cell covers a small geographic area and collectively a group of adjacent cells covers a larger geographic region. Each cell has a fraction of the total amount of RF spectrum available to support cellular users. Cells are of different sizes (for example, macro-cell or micro-cell) and are generally fixed in capacity. The actual shapes and sizes of cells are complex functions of the terrain, the man-made environment, the quality of communication and the user capacity required. Cells are connected to each other via land lines or microwave links and to the public-switched telephone network (PSTN) through telephone switches that are adapted for mobile communication. The switches provide for the hand-off of users from cell to cell and thus typically from frequency to frequency as mobile users move between cells.
In conventional cellular systems, each cell has a base station with RF transmitters and RF receivers co-sited for transmitting and receiving communications to and from cellular users in the cell. The base station employs forward RF frequency bands (carriers) to transmit forward channel communications to users and employs reverse RF carriers to receive reverse channel communications from users in the cell.
The forward and reverse channel communications use separate frequency bands so that simultaneous transmissions in both directions are possible. This operation is referred to as frequency division duplex (FDD) signaling. In time division duplex (TDD) signaling, the forward and reverse channels take turns using the same frequency band.
The base station in addition to providing RF connectivity to users also provides connectivity to a Mobile Telephone Switching Office (MTSO). In a typical cellular system, one or more MTSO's will be used over the covered region. Each MTSO can service a number of base stations and associated cells in the cellular system and supports switching operations for routing calls between other systems (such as the PSTN) and the cellular system or for routing calls within the cellular system.
Base stations are typically controlled from the MTSO by means of a Base Station Controller (BSC). The BSC assigns RF carriers to support calls, coordinates the handoff of mobile users between base stations, and monitors and reports on the status of base stations. The number of base stations controlled by a single MTSO depends upon the traffic at each base station, the cost of interconnection between the MTSO and the base stations, the topology of the service area and other similar factors.
A handoff between base stations occurs, for example, when a mobile user travels from a first cell to an adjacent second cell. Handoffs also occur to relieve the load on a base station that has exhausted its traffic-carrying capacity or where poor quality communication is occurring. The handoff is a communication transfer for a particular user from the base station for the first cell to the base station for the second cell. During the handoff in conventional cellular systems, there may be a transfer period of time during which the forward and reverse communications to the mobile user are severed with the base station for the first cell and are not established with the second cell.
Conventional cellular implementations employ one of several techniques to reuse RF bandwidth from cell to cell over the cellular domain. The power received from a radio signal diminishes as the distance between transmitter and receiver increases. Conventional frequency reuse techniques rely upon power fading to implement reuse plans. In a frequency division multiple access (FDMA) system, a communications channel consists of an assigned particular frequency and bandwidth (carrier) for continuous transmission. If a carrier is in use in a given cell, it can only be reused in cells sufficiently separated from the given cell so that the reuse site signals do not significantly interfere with the carrier in the given cell. The determination of how far away reuse sites must be and of what constitutes significant interference are implementation-specific details.
TDMA Conventional Cellular Architectures
In TDMA systems, time is divided into time slots of a specified duration. Time slots are grouped into frames, and the homologous time slots in each frame are assigned to the same channel. It is common practice to refer to the set of homologous time slots over all frames as a time slot. Each logical channel is assigned a time slot or slots on a common carrier band. The radio transmissions carrying the communications over each logical channel are thus discontinuous. The radio transmitter is off during the time slots not allocated to it.
Each separate radio transmission, which should occupy a single time slot, is called a burst. Each TDMA implementation defines one or more burst structures. Typically, there are at least two burst structures, namely, a first one for the initial access and synchronization of a user to the system, and a second one for routine communications once a user has been synchronized. Strict timing must be maintained in TDMA systems to prevent the bursts comprising one logical channel from interfering with the bursts comprising other logical channels in the adjacent time slots. Space Diversity Combining signals from a single source that are received at multiple spaced-apart antennas is called space diversity. Micro-diversity is one form of space diversity that exists when two or more receiving antennas are located in close proximity to each other (within a distance of several meters for example) and where each antenna receives the signals from the single source. In micro-diversity systems, the received signals from the common source are processed and combined to form an improved quality resultant signal for that single source. Micro-diversity is effective against Rayleigh or Rician fading or similar disturbances. The terminology micro-diverse locations means, therefore, the locations of antennas that are close together and that are only separated enough to be effective against Rayleigh or Rician fading or similar disturbances. The signal processing for micro-diverse locations can occur at a single physical location and hence micro-diversity processing need not adversely impact reverse channel bandwidth requirements.
Macro-diversity is another form of space diversity that exists when two or more receiving antennas are located far apart from each other (at a distance much greater than several meters, for example, ten kilometers) and where each antenna receives the signals from the single source. In macro-diversity systems, the received signals from the single source are processed and combined to form an improved quality resultant signal for that single source. The terminology macro-diversity means that the antennas are far enough apart to have decorrelation between the mean signal levels for signals from the single source. The terminology macro-diverse locations means, therefore, the locations of antennas that are far enough apart to achieve that decorrelation. Since macro-diversity processing involves forwarding of signals to a common processing location, an adverse impact on channel bandwidth tends to result from macro-diversity processing.
Shadow Fading
The decorrelation of mean signal levels employed in macro-diversity systems is due to local variability in the value of signal strength diminution to each of the spaced-apart receiving antennas. This local variability exists on length scales above Rayleigh or Rician fading and is due to terrain effects, signal blocking by structures or vegetation, and any other variability that exists in a particular environment. This variability is referred to as shadow fading. Decorrelation lengths for shadow fading may be as small as length scales just above Rayleigh fading length scales (for example, less than a few meters), or may be as large as several kilometers.
Signal Quality Enhancements
In order for diversity combining to increase the quality of a signal, some measure of the quality of the input signals must be generated. One of the difficult problems in designing space-diversity algorithms is finding an accurate measure of precombination decision reliability, which can be computed in real-time. While micro-diversity systems improve system quality by ameliorating the effects of Rayleigh fading, which is short-term in nature, they are not very effective in combatting shadow fading, which is caused by effects such as an obstruction coming between a transmitter and a receiving antenna. While macro-diversity systems combine received signals from a number of receivers spaced far apart in space to combat shadow fading, in order for macro-diversity combining to increase the quality of the resulting signal, some measure of the quality of the individual received signals is necessary.
In the above cross-referenced application entitled METHOD AND APPARATUS FOR WIRELESS COMMUNICATION EMPLOYING AGGREGATION FOR DIGITAL SIGNALS, a communication system is disclosed having a plurality of forward channel communications and a plurality of corresponding reverse channel communications from and to a plurality of mobile users. A plurality of collectors is distributed at macro-diverse locations for receiving reverse channel signals from the users. Each of the collectors typically includes micro-diversity receivers for receiving the reverse channel signals from users. The collectors forward these reverse channel signals to the aggregators. The aggregators combine the received signals from the macro-diverse collectors. The combining of multiple collector signals for the same user that are both macro-diverse and micro-diverse results in an output bit stream with fewer bit errors.
In one embodiment of that cross-referenced application, the micro-diverse combining occurs in the collectors and the macro-diverse combining occurs in the aggregators. In an alternative embodiment, some or all of the micro-diverse combining occurs along with the macro-diverse combining in the aggregators.
In the aggregation method of the cross-referenced application, the signals from users received at collector antennas are processed to yield one or more sequences of bits and corresponding one or more confidence metrics for each bit. Inputs from the same user through multiple micro-diverse antennas at each collector are combined to reduce errors resulting from Rayleigh and similar disturbances. Signals from the same user are processed to form sequences of bits and corresponding confidence metric vectors from multiple macro-diverse collectors. These signals are combined in an aggregator to reduce errors resulting from shadow fading and similar disturbances. Increasing the number of confidence metric bits (that is increasing the amount of bandwidth) tends to increase the quality of signals (particularly weak signals) while reducing the bandwidth available for other uses (hence reducing the capacity of the system or the quality of other parts of the system). An appropriate balance between reverse channel bandwidth, aggregated signal quality and system capacity is required. The aggregator processes the data from the multiple collectors and combines and decodes the resulting streams to reduce the probability of bit errors. The combining process utilizes the confidence metrics to make a final decision on each bit. The number of bits of data used in the cross-referenced application can be large and hence there is a need to reduce the amount of data allocated to confidence metrics.
In accordance with the above background, the communications problems resulting from interference, noise, fading and other disturbances create a need for improved wireless communication systems which overcome the interference problems and other limitations of conventional cellular systems.